Are DLA abundances biased due to dust obscuration

Identification of DLAs relies on a bright background QSO on whose continuum the strong Lyman-a and metal-line species can imprint their absorption signature. If an intervening galaxy is rich in dust and metals, the very background QSO on which we rely will appear fainter and redder and may 'drop out' of traditional quasar surveys. This idea has been around for more than 20 years (Ostriker & Heisler 1984; Fall & Pei 1993) and it has been suggested that extinction bias may 'hide' more than 70% of gas at high redshift. Four pieces of observational support are often cited for the dust-bias scenario: (1) there is only mild redshift evolution in DLA metallicities, (2) metallicities are low compared with those of emission-line galaxies at similar redshifts, (3) continuum slopes for QSOs with DLAs steeper than those for QSOs without (Pei, Fall & Bechtold 1991) and (4) there is an anticorrelation between N(H i) and metallicity (Prantzos & Boissier 2000), see Figure 22.1.

In order to quantify the impact of dust bias, two surveys based on radio-selected QSOs have now been completed. Together, the Complete Optical and Radio Absorption Line System (CORALS) (Ellison et al. 2001) and the UCSD survey (J0rgenson et al. 2006) cover a redshift path Az ~ 100 for 119 radio-selected QSOs with very deep (and, in the case of CORALS, complete) optical follow-up. In neither survey was an excess of absorbers found either at high or at low (Ellison et al. 2004) redshift and the neutral-gas content is in good agreement (within a factor of 2) with optical samples, see Figure 22.2. More importantly for the present discussion,

Figure 22.2. Mass density of neutral gas in DLAs from SDSS (points shown as a function of redshift) and radio-selected surveys (cross). Figure adapted from J0rgenson et al. (2006).

Figure 22.2. Mass density of neutral gas in DLAs from SDSS (points shown as a function of redshift) and radio-selected surveys (cross). Figure adapted from J0rgenson et al. (2006).

Akerman et al. (2005) have shown that the metallicities of DLAs in the CORALS sample are not significantly higher than those of optically selected samples and do not populate the parameter space at high N(H i) and high metallicity (Figure 22.1). However, since metallicities are usually weighted by the rare high-N(H i) absorbers, a larger sample is required in order to make this result robust.

So, how do we explain the observational 'evidence' in support of dust bias? Authors of a number of recent papers have proposed the idea that, although DLAs may be reservoirs rich in atomic gas, they do not generally flag the location of the bulk of star formation and, therefore, metals (e.g. Wolfe & Chen 2006). Indeed, emission-line spectroscopy of z ~ 0.5 galaxies causing absorption-line systems typically observes Solar abundances (Ellison et al. 2005b). Such observations indicate that high abundances can be found in absorption-selected galaxies, although they may be confined to smaller regions than the cross-section of DLA-producing gas. The cause of the anti-correlation between N(H i) and metallicity is still being debated. However, Ellison et al. (2005 a) have argued that the very low values of reddening that are now being determined for DLAs may soon make dust obscuration an unviable explanation. An alternative explanation may be that sight-lines that pass through high-column-density, high-metallicity gas are simply rare. Simulations support this idea (e.g. Johansson & Efstathiou 2006), showing that the cross-section for such gas at z ~ 3 is small. Finally, concerning the reddening of QSO continua, this may simply have been a case of a difficult measurement combined with small-number statistics. By fitting continua to ~1500 SDSS spectra,

Murphy & Liske (2004) have found reddening to be very low, E(B — V) < 0.02, and a similarly low value has been found by Ellison et al. (2005 a) in CORALS QSOs, E(B — V) < 0.04 based on optical-to-IR colours. Therefore, regardless of concerns about the modest sample size of radio-selected QSO surveys such as UCSD and CORALS, there is no compelling observational evidence to invoke a dust bias. Combined with this revised observational view of dust obscuration, it is interesting to note that theory is also re-assessing the effect of extinction. For example, Trenti & Stiavelli (2006) estimate the total gas density in DLAs to be underestimated by only ~15% in optical surveys.

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